The United States has ordered Nato ally Turkey to stop attacking Kurdish units in northern Syria and focus on fighting the Islamic State (Isis). Turkish troops have pounded areas around Jarablus, despite the town being liberated from IS last week (Tuesday 23 August), a state of affairs the US calls "unacceptable and a source of deep concern".

The latest tensions come as the White House recently announced that US President Obama and Turkish President Erdogan will meet face-to-face at the G20 summit in Hangzhou, China, on Sunday (4 September).

The border town of Jarablus was taken by the Turkish-backed Free Syrian Army (FSA). Ankara says it is clearing both IS and the Kurdish Popular Protection Units (YPG) from the border region.

However, the YPG are one of the main forces within the Syrian Democratic Forces (SDF), which the US regards as allies. The YPG claims to have withdrawn from Jarablus and says Turkey's claims to be attacking Kurdish "terrorists" are a pretext for occupying Syria.

In a series of strongly-worded Tweets on Monday afternoon (29 August), the US special envoy to the coalition, Brett McGurk - quoting from a Defence Department (DOD) briefing - said: "DOD: Monitoring reports of airstrikes & clashes south of #Jarablus b/w Turkish forces, some opposition groups, & units affiliated with #SDF."

McGurk continued: "We want to make clear that we find these clashes - in areas where #ISIL [Isis] is not located - unacceptable and a source of deep concern. The United States was not involved in these activities, they were not coordinated with U.S. forces, and we do not support them.

"Accordingly, we call on all armed actors to stand down and take appropriate measures to deconflict & open channels of communication. The US is actively engaged to facilitate such deconfliction and unity of focus on #ISIL, which remains a lethal and common threat."

Ankara says the YPG is linked to the Turkish PKK, responsible for countless acts of terrorism within its borders. However, Kurd-dominated areas of Turkey have been devastated by Turkish military actions. Turkey is concerned that Kurds within their country, Syria, Iraq and Iran may be trying to move towards an independent state.

Relations between Turkey and the US were already fraught, following the failed coup in July and President Erdogan's overtures to Russia just months after Turkish troops shot down a Russian fighter.

Smoke rises from the Syrian border town of Jarablus as it is pictured from the Turkish town of Karkamis, in the southeastern Gaziantep province on 24 August 2016UmitBektas/Reuters